The use of optical fibers, particularly as a telecommunication transmission medium, has numerous advantages over existing telecommunication media. For example, optical fibers can sustain a broader bandwidth signal and hence, can convey larger quantities of information over the same time period. Further, light waves used in optical fibers have even shorter wavelengths than the conventional microwaves commonly used in telecommunication systems. Thus, a reduction in the physical size of related components is achievable. This size reduction further can result in an overall cost reduction for materials, packaging and manufacturing. Still further, optical fibers exhibit little or no electromagnetic or radio frequency radiation thus resulting in a negligible environmental impact. Conversely, optical fibers are relatively insensitive to electromagnetic and radio frequency interference from the surrounding environment.
To be viable, every telecommunication system must include some means for controllably redirecting a signal, or at least a portion thereof, to or from a transmission medium or between one, or more, such media. In the case of an optical telecommunication system the redirecting means is an optical switch. Currently, optical switches are generally mechanical in nature. Unfortunately, mechanical switches require relatively high driving power and are subject to wear, abrasion and fatigue. As a result, mechanical switches are prone to failure after repeated use. In addition, since a rather small optical fiber is usually displaced from alignment between output port fibers (or input port fibers), mechanical switches can easily become expensive. One particular reason for this expense is the very small tolerances required to ensure the proper alignment between the optical fibers moved and the optical fibers associated with the ports.
Solid-state switches based on an electrooptic or magnetooptic principle have also been used and are inherently more reliable, but these switches are subject to greater crosstalk and insertion loss.
Recently, optical switches have been proposed as an alternative to mechanical switches. Typical of such liquid crystal optical switches are those described in U.S. Pat. Nos. 4,792,212; 4,790,633; and 4,813,796, each to Baker. Baker's patents describe related polarization dependent optical switches. By polarization dependent, it is meant that the switches divide the light signal into two mutually orthogonal and incoherent planes of vibration. Baker's switches include optical fibers 88 attached to the side angled surfaces 24, 32, 70, and 78 of a pair of opposed trapezoidal prisms 20 and 28 and a pair of opposed parallelogram shaped prisms 66 and 74. Each of the four prisms are arranged with their bases parallel to each other and with a passive liquid crystal material 36 and 82 displaced therebetween. Between the top two prisms 20 and 66, and the bottom two prisms 28 and 74, is an active liquid crystal material 50. Electrodes 54 and 56 are coupled to this active liquid crystal material 50 for generation of an electric field by application of a voltage.
Light enters either of two prisms 20 or 28 through either input ports 38 or 40, respectively. By altering the applied voltage to the liquid crystal material 50, the switch controls whether the light will exit the switch through surface 70 or 78. The applied voltage is varied between zero and a finite predetermined value.
By way of example, consider an optical signal entering the switch through side 24 into prism 20. Due to the angle of the surface 24, the light is directed toward the beam splitter 36 which is a passive liquid crystal material (i.e. no voltage is applied to this material). The passive liquid crystal material 36 splits the light into first and second polarized components wherein the first polarized component passes through to the opposing prism 28 while the second polarized component is reflected. The reflected component is reflected to the active crystal material 50. By applying a voltage to the active liquid crystal material 50, the polarization of the second component is reoriented.
The second light component is transmitted to the active liquid crystal material 50 which separates the bottom two prisms 28 and 74 from each other. By applying a voltage to this active material, the polarization of the second component can likewise be altered. The two components are then directed to the passive beam combiner 82 which combines the first and second components as they are directed to the appropriate exit ports located at surfaces 70 and 78.
Because Baker's switches utilize liquid crystal material that is polarization dependent, they resolve light into two mutually orthogonal and incoherent planes of vibration. This resolution of light into two perpendicular planes occurs naturally whenever light reflects off any surface.
FIGS. 1a and 1b illustrate the spacial relationship, in a cartesian coordinate system, between the propagation direction of the light signal as compared with the electric field and the magnetic field in an optical switch. FIG. 1a depicts the electric field perpendicular to the plane of incidence. FIG. 1b has the electric field E in the plane of incidence. Reflectance is not the same for these two polarizations. The electric field vector transverse to the plane of incidence has the larger reflectance.
Two vectors specify an electromagnetic wave, the electric field: E, and the magnetic field: H. The vector K specifies the propagation of the light signal. The vectors E', H' and K' represent the electric field, magnetic field and light signal vectors, except, here they are for the light signal after it has reflected off of the liquid crystal material surface represented by the X-axis. The vectors E", H" and K" represent the electric field, magnetic field and light signal vectors, except, here they are for the light signal after it has passed through the liquid crystal material.
The electric field E and magnetic field H are interdependent. Their space and time derivatives are interrelated in a manner expressed by the Maxwell equations. Changes in the fields E and H propagate a wave through space. The electric and magnetic fields are perpendicular to one another, and perpendicular to the direction of propagation, K. It is traditional in optics to designate the direction of the electric field as the direction of polarization. In the phenomena of reflection and refraction of light, the electric field resolves into components perpendicular and parallel to the plane of incidence (X-Y plane).
The geometry is defined by two other orthogonal planes, the surface plane and the plane of incidence. The plane of incidence contains the light ray. The light ray intersects the surface. The angle the light ray makes with the normal to the surface is the angle of incidence.
FIG. 2 illustrates the angle of incidence .theta..sub.i and angle of reflection .theta..sub.r in relation to the plane of incidence 4 and the interface 2. The interface 2 is sandwiched between Media 1 (the region above the interface) and Media 2 (the region below the interface). Media 1 represents, for example, a top prism while Media 2 represents, for example, a bottom prism. The interface could be a liquid crystal material sandwiched therebetween. Light travelling in Media 1 and in the plane of incidence 4 meets the interface 2 at an angle .theta..sub.i. Part of the light reflects off the interface at an angle .theta..sub.r where .theta..sub.r =.theta..sub.i. The rest of the light enters the Media 2 making a refraction angle .theta..sub.t with respect to the interfacial normal.
The light wave vibrating parallel to the interface has a different reflectivity than light vibrating normally to the surface. The reflected light has slightly more of the perpendicular component. At special angles, all of the reflected light has the parallel polarization. A common example of this can be seen in polarized sunglasses which block out this parallel polarized reflected light thus eliminating glare.
The reflecting surface in all other optical switches is a polarizing beam splitter. The polarizing beam splitter separates light into two planes of polarization. The two planes of polarization are perpendicular to the direction of light propagation. Depending on the state of the beam splitter either one or both polarizations reflect. Transition between these two states constitutes optical switching. For a liquid crystal beam splitter, an electric field forces the change of states. All other optical switches move between these two states by varying the applied voltage from zero to a predetermined finite value. This is illustrated in FIG. 3. When a voltage is applied across the liquid crystal material 12 in the prior art devices, the orientation of the molecules 14 is changed thereby redirecting the optic axis. The optic axis is the longitudinal axis of the molecules within the liquid crystal material. This changes reorients the molecules between orientations perpendicular and parallel to the longitudinal plane of the liquid crystal material 12. This changes the refractive index of the liquid crystal material. When the molecules 14 are perpendicular to the longitudinal axis, as shown in FIG. 3A, the light signal 16 transmits through the liquid crystal material. When the molecules 14 are parallel to the longitudinal axis, however, the light signal 16 is reflected.
Unfortunately, when the molecules are perpendicular to the longitudinal axis, on a portion of the light signal is transmitted through the liquid crystal material. A portion of the light signal is also reflected. This results in a failure to totally switch the light from one exit port to another. When molecules are parallel to the longitudinal plane, both planes of polarization reflect. When the molecules are perpendicular to the longitudinal plane, one polarization reflects; whereas, the other polarization passes through the beam splitter. Therefore, instead of going from 100% on to 100% off, the switch goes from 100% on to 50% on.
Furthermore, the change in states (i.e. from parallel to perpendicular) is not a transition between a totally ON to a totally OFF state. Rather, this is a transition from nearly totally ON to partially OFF. If light had only one rather than two polarizations, then a single beam splitter would switch from totally ON to totally OFF. Other devices eliminate one of the polarizations or have two beam splitters or have a means for converting one polarization into another. It is key to note that all of these optical switches manipulate the light signal's polarization. The left over polarizations at the first beam splitter indicate that all other switches are polarization dependent.
There are many drawbacks to the optical switches described in the Baker patents. The Baker switches have four prisms, two beam splitters and two controlled retardation cells which each require precise alignment for proper light transmission and switching. This requires considerably more effort to build and maintain. The controlled retardation cell also requires precise alignment with regard to the three Eulerian angles in order to minimize crosstalk. The controlled retardation cell introduces additional regions of insertion loss at the cell/prism interface. Minimization of insertion losses at this interface requires antireflection coatings on the prism surfaces. The controlled retardation cells also require precise cell spacing to insure proper phase retardation. Finally, the Baker switch requires three voltage levels to control the switching.
In addition to the four prism design of Baker, a two prism, polarization dependent design is presented in Skinner, J., Lane, C. H. R., "A Low-Crosstalk Microoptic Liquid Crystal Switch," IEEE Journal On Selected Areas In Communications, 6(7):1178-1185 (1988). Skinner describes a polarization dependent optical switch that incorporates two trapezoidally shaped glass prisms of equal refractive index n.sub.g and with a base angle A. The inside faces of the prisms are coated with a transparent electrode (commonly indium tin oxide) and a thin polyamide layer.
The polyamide layers are rubbed in antiparallel directions along the y-axis to induce subsequent OFF state alignment of the liquid crystal (homogenous alignment). The prisms are then bonded together using an epoxy edge seal loaded with spacers of the desired diameter (typically a few microns). Liquid crystal is introduced into the space between the prisms by vacuum filling, and the cell is sealed.
The principle of operation includes an ON state wherein the light is transmitted and an OFF state wherein the light is reflected. In the ON state, a voltage is applied between electrodes, and the liquid crystal is aligned normal to the prism faces, so that "p" polarized incident light sees a refractive index approaching n.sub.e ' a value near the greater of the two liquid crystal indexes. The prism index and base angles are chosen such that: EQU n.sub.g =n.sup.1.sub.e '
and ##EQU1## where n.sub.o and n.sub.e are the ordinary and extraordinary indexes of the liquid crystal, respectively. In this case, the polarized light encounters no optical discontinuity at the liquid crystal sandwich, and is entirely transmitted along path B.
In the OFF state, the liquid crystal molecules tend to align with their long axes parallel with the direction of the microscopic scratches formed by rubbing the thin polyamide layer. In this case, the polarized input light sees a refractive index n.sub.0.sup.1 and if n.sub.g and A are chosen such that: ##EQU2## where: ##EQU3## then the light undergoes total internal reflection, and is reflected along path A.
A problem with Skinner is that it operates with a 3 dB polarization loss. This is because it discards the perpendicular polarization (i.e. it is polarization dependent). By throwing away one of the polarizations (i.e. 50% of the power transmission) 50% of the input intensity is lost. This becomes of critical importance when the light signal travels through a switching network. The result is a limitation on the number of switches possible in the network.
Furthermore, the Skinner switch operates between a powered ON and a powered OFF state. This results in bounce which describes diminishing ON and OFF cycles as the power is turned off. Furthermore, optimum performance occurs at 100 Volts p/p. This is due to the ON state field driving the molecules from a parallel to a perpendicular alignment. Only at very high field strengths is this possible.
Another polarization dependent optical switch is presented by Meadows, M. R., et al., "Electro-optic Switching Using Total Internal Reflection By A Ferroelectric Liquid Crystal," Appl. Phys. Lett., 54(15):1394-1396 (1989). Meadows describes another polarization dependent optical switch that incorporates two trapezoidally shaped prisms with their bases positioned opposite each other. An optically transparent electrode covers the two bases and sandwich a ferroelectric liquid crystal (FLC) film. An applied DC electric voltage of approximately 10V/.mu.m of film thickness selects between two optic axis orientations, both lying nearly parallel to the plane of the film, but differing by up to 90 degrees. The polarity of the applied field determines which orientation is selected. If the optic axis is switched to the second direction, the light is decomposed into an ordinary ray which sees the principle refractive index n.sub..omega. and an extraordinary ray which sees the refractive index. ##EQU4## Where .theta. is the angle of incidence. Common FLCs are positive uniaxial with n.sub..epsilon. .gtoreq.n.sub.e &gt;n.sub..omega. so it is possible to choose .theta. to be beyond the critical angle for both rays. This requires: ##EQU5## and causes the incident light to experience total internal reflection (TIR). This is the OFF state.
When p-polarized light is incident, the electric field is perpendicular to the optic axis in the ON state: the light experiences the index n.sub..omega. and TIR. In the ON state, the light gives rise to the same two rays as does the s-polarized light, and also experiences TIR. Thus, p-polarized light is reflected in both the ON and OFF states.
The resulting switch switches the s polarized light component of the incident light between transmission and reflection while always reflecting the p-polarized light component.
In the fabrication process, one of the two optic axis directions is made perpendicular to the plane of incidence of the light to be switched. The other optic axis direction is still in the plane of the film but is rotated by the angle .psi.. Therefore, supposing that the applied field selects the perpendicular direction. When s linearly polarized light falls on the film, its electric field is parallel to the optic axis for the light becomes an extraordinary ray which sees the principle refractive index n.sub..epsilon., then the light will cross the FLC film. This is the ON state.
A drawback with Meadows' switch is that in the ON state, it is a beam splitter. This means that the p-polarization reflects while the s-polarization transmits through the device. Therefore, there is no total transmission of the light signal. Furthermore, the Meadows switch utilizes a smectic ferroelectric liquid crystal film. A drawback of this film is that it is difficult to remove domain imperfections which contribute to crosstalk. Still further, the optic axis of the liquid crystal film rotates in a plane parallel to the film layer. Rotation in the plane parallel to the film layer does not permit full on and off switching. Furthermore, rotation in a plane requires interdigitated electrodes or ferroelectric liquid crystal material. Interdigitated electrodes are on the same inner glass surface. These electrodes develop a field within the same surface rather than between the two inner glass surfaces. The field generated by the interdigitated electrodes twists the optic field parallel to the glass surface. It is difficult to form a single domain with ferroelectric liquid. Zig-zag walls and imperfections in the ferroelectric liquid crystal are difficult to avoid. The alignment of the smectic layer changes discontinuously across a zig-zag wall producing refractive index discontinuities and poor extinction.
A double-pass optical switch is described in Soref, Richard A., McMahon, D. H., "Total Switching of Unpolarized Fiber Light With a Four-Port Electro-Optic Liquid-Crystal Device," Optical Society of America, 5(4):147-149 (1980). The optical switch described by Soref also includes two trapezoidally shaped glass prisms having a transparent electrode layering their bases. The bases then sandwich a liquid crystal material therebetween.
To accomplish total switching, it is necessary to align the nematic liquid crystal molecules in the plane of the layer at 90 degrees to the light propagation direction for the V=0 state. This is in contrast to previous devices wherein the liquid crystal material's optical axis coincided initially with the propagation. The double-pass switch also requires internal reflection of both polarizations. This is accomplished by specular reflection at glass-air interfaces.
Soref's switch uses unpolarized light as the input signal. However, it processes the input signal by separating it into two orthogonal polarizations. These polarizations are launched into the switch's wave guides, selected, and then recombines them to form an unpolarized light signal. The selection event decides whether to convert or not convert one polarization into the other.
Meadows in the third paragraph cites authors, Krashnow, and Soref. Krashnow at General Electric was the first to publish a paper using liquid crystal in a switch. The Krashnow switch used polarized light. Soref modified the Krashnow design to use unpolarized light as input. Soref did this by designing a switch with two optical components. The first optical component, a beam splitter broke light into two polarizations. The second component, beam combiner recombined the polarizations. This exemplifies that Soref's switch is polarization dependent.
What is therefore needed is an efficient optical switch that resolves the drawbacks described above.